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U2FsdGVkX1+gU3wqsvthBA4fJYt+YFJhDhkq8g+XHDV/YOsxUQI17Z7FsvFXOjZaT3GnHIkFT3eDGgjaHW1v8IQVpMnsofbptnt4aEC0IFRP8EJOKKCDUdCDX3NWtog39Wi6s+UB3oRb9w043DK/a+YYwocbJ0fyB09J7nMDzyCltUrrNqdSyxEazRG9+KmFTBX8Wch/wPUM+kCCv7ZVQh1hwA8x9Iw2Bsjd9QuiUH1KsylegXLfofTMnozYb/NVbRLmchb/Qvh8EgJEYAhrdysavncvHZXe9P+d5uyff7f+7EBvZknTKOhXJ4Qs7n8OV+lyJjv8VZmsMVKuGZK9XkZLItXoP97Poavy+egYpw56bLqSiPaxb+l/gg5jzbzb/rXmd+XDKbcvSCRaF/LK91Pj1ILma9+7IW/3Q0LH1M+RV0kCxhNu/LQjiMslvXpJ68GiKkhQXigdMpOOzSHVkX1ppVomW9Al/HjS3WhDfY9kaDgf8Sh2FhxjanAGVJDMhb0yhYKTUc0ohbjyjiT8UhAEjSV6vfjt8xACszYzvdaWQctfbByFT5iyxolGPjwu8eZmnQW4SFKLzYWg45QY971bl8LYt1//TnCidBjlG1W5S5HlRWVDORPeK5JZjJjqQmXSSuXzVg6sxcBDV105Mnkpx/qm9UQC5v9YdykZWALBfOBuPrtuNU2InL7agBqayUoQrvbC6I6HBbzyBj+8eQGqeM6KijrLPBmtFjcrugtG6pAIn+qoiCutx1qilB/b5LTgzjDo8m1PdbBZlMdXv7lXkIexDFmeWpeC2mwq0obOyiHIkaR794BSxotRkgqviPvXw27fro+Kcxysqo4ovh654c8GTj4NiixW/5/hev+ydjH9kqYuGjLt4YPsqnZBvs6qvgtbswPdTLSHyq7IpbQnDcJ9q9ibbObtP477xpk2fQ/0TW6Yz8S0WFPzQfYWpFSw2yLbe4bc+ONji2+ Qqv73MO/ 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1/13/26 3:08 AM, Harry Yoo wrote: > > On Mon, Jan 12, 2026 at 04:16:55PM +0100, Vlastimil Babka wrote: > >> After we submit the rcu_free sheaves to call_rcu() we need to make sure > >> the rcu callbacks complete. kvfree_rcu_barrier() does that via > >> flush_all_rcu_sheaves() but kvfree_rcu_barrier_on_cache() doesn't. Fix > >> that. > > > > Oops, my bad. > > > >> Reported-by: kernel test robot > >> Closes: https://lore.kernel.org/oe-lkp/202601121442.c530bed3-lkp@intel.com > >> Fixes: 0f35040de593 ("mm/slab: introduce kvfree_rcu_barrier_on_cache() for cache destruction") > >> Cc: stable@vger.kernel.org > >> Signed-off-by: Vlastimil Babka > >> --- > > > > The fix looks good to me, but I wonder why > > `if (s->sheaf_capacity) rcu_barrier();` in __kmem_cache_shutdown() > > didn't prevent the bug from happening? > > Hmm good point, didn't notice it's there. > > I think it doesn't help because it happens only after > flush_all_cpus_locked(). And the callback from rcu_free_sheaf_nobarn() > will do sheaf_flush_unused() and end up installing the cpu slab again. I thought about it a little bit more... It's not because a cpu slab was installed again (for list_slab_objects() to be called on a slab, it must be on n->partial list), but because flush_slab() cannot handle concurrent frees to the cpu slab. CPU X CPU Y - flush_slab() reads c->freelist rcu_free_sheaf_nobarn() ->sheaf_flush_unused() ->__kmem_cache_free_bulk() ->do_slab_free() -> sees slab == c->slab -> frees to c->freelist - c->slab = NULL, c->freelist = NULL - call deactivate_slab() ^ the object freed by sheaf_flush_unused() is leaked, thus slab->inuse != 0 That said, flush_slab() works fine only when it is guaranteed that there will be no concurrent frees to the cpu slab (acquiring local_lock in flush_slab() doesn't help because free fastpath doesn't take it) calling rcu_barrier() before flush_all_cpus_locked() ensures there will be no concurrent frees. A side question; I'm not sure how __kmem_cache_shrink(), validate_slab_cache(), cpu_partial_store() are supposed to work correctly? They call flush_all() without guaranteeing there will be no concurrent frees to the cpu slab. ...probably doesn't matter after sheaves-for-all :) > Because the bot flagged commit "slab: add sheaves to most caches" where > cpu slabs still exist. It's thus possible that with the full series, the > bug is gone. But we should prevent it upfront anyway. > The rcu_barrier() in __kmem_cache_shutdown() however is probably > unnecessary then and we can remove it, right? Agreed. As it's called (after flushing rcu sheaves) in kvfree_rcu_barrier_on_cache(), it's not necessary in __kmem_cache_shutdown(). > >> mm/slab_common.c | 5 ++++- > >> 1 file changed, 4 insertions(+), 1 deletion(-) > >> > >> diff --git a/mm/slab_common.c b/mm/slab_common.c > >> index eed7ea556cb1..ee994ec7f251 100644 > >> --- a/mm/slab_common.c > >> +++ b/mm/slab_common.c > >> @@ -2133,8 +2133,11 @@ EXPORT_SYMBOL_GPL(kvfree_rcu_barrier); > >> */ > >> void kvfree_rcu_barrier_on_cache(struct kmem_cache *s) > >> { > >> - if (s->cpu_sheaves) > >> + if (s->cpu_sheaves) { > >> flush_rcu_sheaves_on_cache(s); > >> + rcu_barrier(); > >> + } > >> + > >> /* > >> * TODO: Introduce a version of __kvfree_rcu_barrier() that works > >> * on a specific slab cache. -- Cheers, Harry / Hyeonggon